Some ink printing devices use a single printhead, but many use a plurality of printheads. For example, seven printheads may be arranged in two rows with one row having three printheads and one row having four printheads. The printheads in the first row are separated by gaps having a distance approximately equivalent to the width of a printhead. The printheads in the second row are positioned to align with the gaps between the printheads in the first row. This arrangement is called a staggered full width array (SFWA) printhead assembly and an exemplary embodiment of a SFWA assembly is shown in FIG. 1.
Synchronizing the passage of an image receiving member with the firing of the inkjets in the printheads enables a continuous ink image to be formed across the member in the direction perpendicular to the direction of member passage. Alignment of the ink drops ejected by the printheads, however, may not be as expected. Each printhead in the printhead assembly has up to three degrees of positional freedom. The printheads need to be precisely aligned to provide a smooth transition from the ink drops ejected by one printhead to the ink drops printed by the other printheads in the assembly. Misalignment of printheads may occur from, for example, printheads failing to meet manufacturing tolerances, thermal expansion of the printhead and associated parts of the printer, vibration of the printhead, or the like.
The possible degrees of movement for a printhead are now discussed with reference to FIG. 2. In general, a printhead 204 that is not attached within a printhead assembly is rotatable about each axis of an XYZ set of axes as shown in the figure. Rotation about the Y axis is called yaw, rotation about the X axis is called pitch, and rotation about the Z axis is called roll. Additionally, in general, a printhead 204 that is not attached within a printhead assembly is also translatable along any one of the axes. These rotations and translations constitute the possible six degrees of freedom of movement for a printhead that is not attached within a printhead assembly. A printhead that is attached within a printhead assembly, however, only has three degrees of freedom, X, Y, and roll, due to fixation of the printhead within the printhead assembly. Changes in printhead position may result from factors such as mechanical vibrations and other sources of disturbances on the machine components, which may alter printhead positions and/or angles with respect to an image receiving surface.
Misalignments between printheads in three of the six degrees of freedom may be categorized as roll or stitch errors. Roll errors can occur when a printhead rotates about an axis normal to the imaging member. Roll error causes a skew in the rows of ink drops ejected by the printhead relative to the imaging member. This skew may be noticeable at the interface between two printheads and may cause an objectionable streak. Stitch errors arise when the printhead shifts in the process (Y) direction or the cross-process (X) direction. These errors result in misalignment of drops from one printhead with the drops of another printhead.
In the case of Y-direction stitch errors, the drops in the rows of one printhead are shifted up or down from the drops in the rows of a neighboring printhead. In the case of X-direction stitch errors, the first and last drops in the rows printed by the shifted printhead are too close or too far from the last and first drops, respectively, in the rows printed by the neighboring printheads. Of course, if the shifted printhead is the first or last printhead in the assembly, shifting of the first drop or the last drop in the rows, respectively, does not occur at an intersection with another printhead. Thus, aligning printheads in a printhead assembly with sufficient accuracy to allow high image quality is desired.
To adjust the printhead position to correct for printhead misalignments, one previously known printhead assembly uses a right-angle lever arm that pivots to push a plate to which a printhead is mounted. The movement of the printhead in this assembly is limited by the length of the lever arm. Additionally, movement of the lever arm may produce backlash and compensation for this backlash must be incorporated in the movement of the arm to properly adjust the position of the printhead. Printhead adjustment systems that operate with little or no backlash and that possess an increased range of movement are desirable.